The invention relates to a powder coating which has improved surface properties through the use of a levelling agent.
For some years, powder coatings have been finding an ever greater field of application. Powder coatings are in the form of a dry powder which, generally with the aid of electrostatic charging, is applied in finely divided form to the surface of the article to be coated and is then dried, i.e. stoved. In the course of stoving, which is usually effected at temperatures between 130 and 220.degree. C., crosslinking reactions and film formation give rise to the solid coating on the workpiece. Powder coatings of this kind offer a range of advantages. For instance, the pulverulent consistency of the coating material does away with the need to operate with solvents. Accordingly, powder coatings are unhampered by all of those environmental problems commonly associated with the evaporation of the solvents. Moreover, the heat energy otherwise necessary to evaporate the solvents is spared. A further advantage of powder coatings is that material which has not adhered permanently to the workpiece can be collected at the bottom of the powder booth and used again without problems. In this way it is possible to achieve a degree of powder utilization of up to 98%. This increased degree of utilization brings not only cost advantages but also a much lower level of environmental pollution than is the case with conventional coating materials.
Powder coatings are prepared from binders, hardeners, fillers, pigments and additives. In terms of the binders and hardeners used, modern-day powder coatings are generally based on one of the following polymer classes: epoxides; epoxy/polyester mixtures; polyesters; polyesters/isocyanates (polyurethanes); acrylates.
One problem associated with the use of powder coatings, however, is that of obtaining a surface having sufficiently good properties. Thus, in general, considerable defects appear in the stoved powder coating film which reduce the surface smoothness. One of the causes of these surface defects is the fact that the powder of the coating material, applied in a nonuniform manner and with its own structure, does not have sufficient time in the course of stoving to flow out during its liquid phase (with a minimal viscosity level) and in doing so to form a uniform covering of the surface of the workpiece. This is because melting and the attainment of minimum viscosity are accompanied simultaneously by the beginning of crosslinking of the binders and therefore of polymerization of the coating material. A further important cause of irregular film surfaces are impurities in the powder coating material.
To improve the surface smoothness of powder coating films, additives have long been employed. For such purposes use is preferably made of acrylate heteropolymers and homopolymers in the molecular weight range of Mw=5000 to 200,000. Typical examples of such polymers are Acronal 4 F.RTM. and Byk-360 P.RTM. (Acronal 4 F.RTM. is a trade mark of BASF AG; Byk-360 P.RTM. is a trade mark of the company Byk-Chemie).
Acronal 4 F.RTM. is a poly-n-butyl acrylate. According to its specification this additive is solvent-free and has a nonvolatiles content of less than 1.5% at 30 min/140.degree. C. It is a soft, viscous, tacky resin which is difficult to hydrolyse and has excellent light and ageing stability. Used alone, Acronal 4 F.RTM. produces a tacky film of low strength.
Byk-360 P.RTM. is an acrylate polymer applied to a specific silicic acid. The active substance is compounded at 60% to form a white, flowable powder. This levelling additive is equally effective in epoxy-, acrylate-, polyester- and polyurethane-based powder coatings. It is mixed with resin, hardener and pigments and then extruded with all of the components.
Occasionally, low molecular mass polyesters are also used. However, the results obtained accordingly do not meet the requirements made of the surface. Moreover, such surfaces possess in the unsanded state a degree of overcoatability which is often unsatisfactory.
The coating of car wheels made of steel or aluminium is a multicoat system. In such a system the powder coating is one constituent and may be employed in the systems as a priming powder coating, a metallic powder coating or a transparent powder coating. Depending on the specification or design, liquid coating material and powder coating material are combined in the system. In each of these cases, an extremely good bond between the individual coats is required, whether between the powder coats themselves or between powder coating and liquid coating material. The coat system comprising priming powder plus basecoat and clearcoat has proved particularly suitable.
At least equally high requirements are placed on the coating of a car body. In this case, powder coating material is likewise employed as primer or as topcoat. In the region of the bonnet in particular, a critical factor is particularly good, smooth levelling coupled with extremely good intercoat adhesion in the vulnerable stone chip zone.